The present invention relates to a pitch having superior properties as a starting material for the production of carbon fibers.
In recent years there have been reported a number of methods for preparing carbon fibers from pitches.
For example, it has been reported that a carbon fiber having improved elastic modulus and strength is obtainable by heat-treating a commercially available petroleum pitch to obtain a pitch containing an optically anisotropic liquid crystal called mesophase, using this mesophase-containing pitch as a precursor pitch (the pitch used in melt spinning will be hereinafter referred to as "precursor pitch"), then melt-spinning the precursor pitch, rendering the resultant pitch fiber infusible, followed by carbonization and subsequent graphitization if required (see Japanese Patent Laying Open Print No. 19127/1974).
However, many of commercial petroleum pitches and other synthetic pitches, when heat-treated for preparation of the precursor pitch, allow production of high molecular weight components insoluble in quinoline. More particularly, heat treatment of these pitches causes both a thermal decomposition and a polycondensation reaction, whereby low molecular weight components gradually increase in their molecular weight and become high molecular weight components insoluble in quinoline, and at the same time originally high molecular weight components are further increased in their molecular weight. The softening point of the pitch also rises. The presence of a large amount of such quinoline insolubles and the high softening point exert a bad influence at the subsequent melt spinning step. That is, for melt-spinning the precursor pitch, it is necessary to raise the spinning temperature until the viscosity of the precursor pitch becomes a spinnable viscosity. In this case, if the softening point of the precursor pitch is too high, it is inevitably required that the spinning temperature be also raised to the same degree. As a result, not only quinoline insolubles further increase in their molecular weight but also there occurs a thermal decomposition of the pitch to produce a light gas, thus making it impossible to obtain a uniform precursor pitch, that is, it actually becomes impossible to effect spinning of the pitch.
Thus, the precursor pitch must have a relatively low softening point and a viscosity suitable for spinning. Further, it must be substantially free from volatile components during spinning and also during carbonization.
In order to satisfy the above-mentioned requirements, there have been adopted such means as pressure filtration and solvent fractionation for removing the produced quinoline insolubles to thereby prepare a precursor pitch to use for the production of carbon fibers (see Japanese Patent Laying Open Print Nos. 9804/1972, 142820/1975, 1342/1980 and 5954/1980). However, the use of such means requires a more complicated treating apparatus and an increased treating cost, and thus is not desirable from the economic point of view.